One of the currently most important developments in the field of networks is the evolution of data networks for the transmission of real-time traffic, i.e. voice, video information and audio information. To render a data network capable of real time operation, mechanisms must be provided which ensure that so-called quality-of-service features such as, e.g. the transmission duration, the so-called jitter and the packet loss rate are maintained. In a real-time-capable network, it must be prevented that situations occur in which committed quality-of-service features can no longer be guaranteed.
Conventional data networks have the handicap that overload situations can occur which can lead to long packet delays or even packet losses. For this reason, real-time-capable data networks or packet networks—conventional data networks are based on the transmission of IP packets such as, e.g. the Internet, as a rule—work with traffic restriction at least for real-time traffic and prioritization of the real-time traffic in order to be able to provide the required quality of service for the real-time traffic. Traffic restriction is in most cases based on access controls in the course of which traffic to be transmitted is registered and allowed or rejected depending on the available bandwidth. When access controls are set up, a service-compatible quality of service should be guaranteed, on the one hand, and on the other hand, the network operator is interested in transmitting as much traffic as possible in order to achieve the highest possible revenue. When limits are chosen, a compromise must be found, therefore, which allows the transmission of as much traffic as possible without this impairing the quality of service.
In data networks, the operator additionally has the capability of optimizing the transmission or distribution of traffic in his network by specifying so-called metric sets. A metric set is here intended to mean the totality of all “interface cost” parameters configured in the network elements (routers) of the network. Specifying limits for the access control and metric sets presents a considerable expenditure in larger networks. If it is additionally intended for parameters to be predeterminable at the operator side in order to be able to, for example, conform to economic agreements and if the adjustments are intended to be correctable in the case of changed traffic conditions, the operator is confronted with configuring of considerable effort and considerable complexity.
In IP networks, the routing tables are then determined in the routers with the aid of distributed or replicated routing algorithms. The length of the various possible paths to a destination is determined as the sum of the link or interface cost metrics on the paths. The next node of the shortest path is finally entered as “next hop” in the routing table for the corresponding destination. If several equally long shortest paths are determined, the equal cost multiple path (ECMP) option of OSPF can also be used for determining that all corresponding next hop routers are included in the routing table.
To divert the traffic from links with high traffic utilization to less severely loaded links, it has been variously proposed to select all link or interface cost metrics in the network not to be all equal but to adjust them in accordance with the result of a multidimensional optimization
[B. Fortz, M. Thorup, “Internet Traffic Engineering by Optimizing OSPF Weights”, Proc. IEEE Infocom 2000, http://www.ieee-infocom.org/2000/papers/utilization65.ps]. However, this does not guarantee that these optimized metrics will still lead to lower link utilization than the standard adjustment with homogenous metrics on all links even in the case of errors such as e.g. a link failure or a router failure.
As a rule, cost metrics are all set to the same value or to the inverse of the bandwidth in a network or are optimized for the good case (without link failures). In [A. Nucci et al. “IGP Link Weight Assignment for Transient Link Failures”. Proceedings of ITC 2003, Berlin, Germany, September 2003, pp. 321-330] an optimization taking into consideration link failures for a taboo search method for single-path routing is described. With the aid of these cost metrics, the routing protocols determine shortest routing paths between input and output network nodes in the sense of the routing paths with the least cost sum of the connecting paths. If there are several equally good routing paths or routing path sections, i.e. part-sections of the routing path, the data packet traffic or the data packets are distributed over several equally good paths, for example with the aid of the equal cost multipath method (ECMP) for short.
If then an operator of an IP network wishes to (temporarily) deactivate a network element, i.e. for example one or more lines or else an entire node, from his network, he must ensure that no further traffic is handled via this network element if he does not wish to avoid the loss of data packets and thus connection faults. In this respect, he has basically several options:
a) Using an equivalent circuit on the physical layer (layer 1) he can shut down, for example, a line in the field of long distance lines, and another
line takes over its traffic. This procedure cannot be applied if the operator wishes to shut down the complete line from one router to another router or if he wishes to exchange an interface card in a router.
b) In most cases, the method is selected to simply switch off a connection and to leave it to the fault detection system to detect this and then to find a new path in the routing protocol. To avoid relatively large impairments of traffic, this procedure is therefore selected, as a rule, in low-traffic times, that is to say normally during the night.
c) To avoid the disadvantage of option b), namely the sometimes very long time interval until the artificially generated failure is detected, the respective IP interfaces can be deactivated with “interface down” in the two routers affected. This saves the time for error detection and rerouting can start immediately.
d) An alternative to option c) can also consist in setting the cost metrics (interface cost metric) for the connection affected so high at both ends that the connection is no longer used for traffic after a subsequent rerouting. After that, the connection can be taken out of operation without requiring further rerouting.
In all the abovementioned cases, the network operator lacks a robust statement about the extent of the impairment of the traffic and the capability of optimizing the routing in the network towards the new situation. In addition, to operate the network case by case with admission control budgets for securing QoS attributes, the authorization boundaries (budgets) are no longer correct after the deactivation of network components. Should it already have been a matter of admission control with a fault tolerance, no traffic is impaired after the deactivation of a connection but the new fault tolerance must only be established again by corresponding adaptations of the admission control budget in response to the deactivation.